1. Field of Invention
The present invention is related to the manufacture of III-V light emitting and laser diodes, particularly towards improving the characteristics of the electrical contact to the p-type portion of the diode.
2. Description of Related Art
Gallium nitride (GaN) compounds have wavelength emissions in the entire visible spectrum as well as part of the UV. FIG. 1 illustrates a typical GaN-based light emitting diode (LED). Currently, most GaN-based LEDs are epitaxially grown on a sapphire or silicon carbide (SiC) substrate. A double hetero-structure that includes a nucleation layer, n-type layer, active region, p-type AlGaN layer, and a p-type layer of GaN is formed on the substrate. In general, the ability to fabricate ohmic contacts to the p-type layer is essential for the realization of reliable light emitting diodes and laser diodes. Ohmic contacts to p-type GaN are difficult to achieve because the attainable hole concentration is limited for Mg-doped III-nitride based semiconductors. In addition, many light-emitting diodes and vertical cavity surface-emitting laser diodes use thin, transparent metal contacts. The choice of metals is limited and metal layers need to be thin, e.g. <15 nm, to reduce light absorption. Because there is poor lateral current spreading in p-type GaN, the metal layers typically cover nearly the entire device area.
P-type conductivity for GaN is achieved by doping with Mg, which substitutes for gallium in the GaN lattice and acts as an acceptor (MgGa). MgGa introduces a relatively deep acceptor level into the band gap of GaN. As a consequence, only ˜1% of the incorporated Mg acceptors are ionized at room temperature. To illustrate, a Mg concentration ([Mg]) of ˜5e19 cm−3 is needed to achieve a room temperature hole concentration of ˜5e17 cm−3. Further, Mg-doped GaN requires a post-growth activation process to activate the p-type dopants. The post-growth activation process may be, for example, thermal annealing, low-energy electron-beam irradiation, or microwave exposure. For conductivity-optimized Mg-doped GaN layers, [Mg]<5 e19 cm−3, the acceptor concentration (NA) is about equal to the atomic Mg concentration and the resistivity can be around 1 Ωcm or less. These layers may be referred to as “p-type conductive layers”. Increasing the Mg content beyond approximately 5e19 cm−3 does not translate to higher acceptor concentration. Typically, a reduction of NA is observed when the [Mg] exceeds a certain maximum concentration and the layer becomes resistive.